1. Field of the Invention
The present invention relates to a power amplifier module, and in particular to a power amplifier module having the function of preventing an amplifier from being damaged due to overcurrent.
2. Description of the Background Art
Generally, in order to emit into space a carrier wave having information superimposed thereon, a wireless apparatus includes a power amplifier module for amplifying a signal to a sufficient level; and an antenna for emitting the carrier wave.
In the antenna, in order to efficiently transmit and receive signals, impedance matching is done in the transmit and receive frequency band. By thus doing matching, an output signal from the power amplifier can be emitted from the antenna. That is, reflected power is not inputted to the power amplifier module from the antenna.
However, if the antenna is damaged or broken for any reason, the matching is deteriorated. As a result, part of an input signal to the antenna from an output terminal of the power amplifier becomes reflected power, and the reflected power is inputted to the output terminal of the power amplifier. In this manner, if reflected power is inputted to the power amplifier, the reflected power is superimposed on the power of a transmit signal. Part of the power which includes the reflected power is consumed as heat by the power amplifier. As a result, the power amplifier causes thermal runaway due to an abnormal temperature increase and then causes overcurrent, whereby the power amplifier may be damaged.
The gain, output, and consumption current of the power amplifier are determined by the load conditions of the power amplifier. If the matching conditions of the antenna are changed by a reflected wave, the load conditions of the power amplifier change as well. Accordingly, due to the generation of the reflected wave, the impedance matching of the power amplifier changes and thereby overcurrent occurs, and as a result, the power amplifier may be damaged.
In order to prevent the power amplifier from being damaged, there exists a power amplifier module which detects, upon occurrence of overcurrent, an abnormal current increase in the power amplifier and suppresses the overcurrent. Specifically, a resistor is inserted in series with a power supply terminal of a power amplifier included in the power amplifier module. The voltage drop generated across the resistor is detected, and the consumption current of the power amplifier is detected. Then, based on the detected consumption current, the consumption current of the power amplifier is controlled (see, for example, Japanese Laid-Open Patent Publication No. 09-199950). The above-described power amplifier module will be described below with reference to the drawing. FIG. 14 is a diagram illustrating a configuration of the power amplifier module.
The power amplifier module shown in FIG. 14 includes a power amplifier 101, a terminal 102, a switching circuit 103, a resistor 104, a drain current detection section 105, a gate voltage control section 106, a loop filter 107, a comparator 108, a terminal 109, a latch circuit 110, and a reset signal circuit 111. The circuit configuration of the power amplifier module will be described below.
The power amplifier 101 has a drain terminal D, a gate terminal G, and a source terminal S. The source terminal S is grounded. The loop filter 107 is connected to the gate terminal G. The resistor 104 is connected to the drain terminal D.
A power supply voltage Vdd is inputted to the switching circuit 103. One end of the resistor 104 is connected to the output side of the switching circuit 103, and the other end is connected to the drain terminal D. The drain current detection section 105 is connected to both ends of the resistor 104. Further, the drain current detection section 105 is connected to an input terminal of the gate voltage control section 106 and to an input terminal of the comparator 108. The gate voltage control section 106 is connected to the loop filter 107. The terminal 109 is connected to an input terminal of the comparator 108. An output terminal of the comparator 108 is connected to the latch circuit 110. The latch circuit 110 is connected to the reset signal circuit 111 and to the switching circuit 103. The roles of the components of the power amplifier module having the above-described circuit configuration will be described below.
The power amplifier 101 comprises a field-effect transistor (FET), for example, and amplifies a signal inputted from the terminal 102 with an amplification factor based on the magnitude of a drain current inputted to the drain terminal D, and then outputs the amplified signal to an antenna (not shown) The resistor 104 is a resistance element having a known resistance value. The drain current detection section 105 detects the voltage drop across the resistor 104 to detect the magnitude of the drain current, and then outputs a signal having a voltage based on the magnitude of the drain current.
Based on the voltage of the signal outputted from the drain current detection section 105, the gate voltage control section 106 controls the gate voltage of the power amplifier 101 through the loop filter 107. By this, an increase in the gate current of the power amplifier 101 is prevented.
The comparator 108 compares between the voltage of the signal outputted from the drain current detection section 105 and a reference voltage to be inputted from the terminal 109, and then outputs a comparison result. Specifically, if it is determined that the voltage of the signal outputted from the drain current detection section 105 is higher than the reference voltage, the comparator 108 outputs a low-level signal. On the other hand, if it is determined that the voltage of the signal outputted from the drain current detection section 105 is lower than the reference voltage, the comparator 108 outputs a high-level signal. The latch circuit 110 holds the signal outputted from the comparator 108. The reset signal circuit 111 outputs a reset signal to the latch circuit 110 to reset the latch circuit 110. The latch circuit 110 outputs the signal being held thereby to the switching circuit 103. If a low-level signal is outputted to the switching circuit 103 from the latch circuit 110, the switching circuit 103 switches from a conduction state to a blocking state.
The operation of the power amplifier module configured in the above-described manner will be described below. The following describes the operation of the power amplifier module performed when reflected power from the antenna is increased and thereby the drain current of the power amplifier 101 is increased.
In the case where the drain current of the power amplifier 101 is increased, the drain current detection section 105 detects an increase in drain current based on the voltage drop across the resistor 104. The gate voltage control section 106 controls the gate voltage of the power amplifier 101 through the loop filter 107. By this, an increase in drain current is inhibited.
Now, the case is described where the drain current is increased despite the control by the gate voltage control section 106. In this case, the signal level outputted from the drain current detection section 105 is higher than the reference voltage inputted from the terminal 109. Hence, the comparator 108 outputs a low-level signal. The latch circuit 110 acquires the low-level signal and then outputs the low-level signal to the switching circuit 103. Accordingly, the switching circuit 103 switches from a conduction state to a blocking state. That is, the drain current becomes zero. Thereafter, the latch circuit 110 is reset by the reset signal circuit 111.
By performing such operation, the conventional power amplifier module prevents the power amplifier 101 from being damaged. In power amplifier modules as disclosed in Japanese Laid-Open Patent Publications No. 09-284062 and No. 11-355054 too, a resistor is inserted in series between a power supply terminal and a power amplifier, and the current is detected through the voltage drop across the resistor.
The above-described conventional power amplifier modules, however, have the problem of an increase in consumption current, which will be described in detail below. In the above-described conventional power amplifier modules, the resistor is inserted in series with the power supply terminal, and the voltage generated across the resistor is detected. Therefore, the voltage dropped by the inserted resistor is applied to the power amplifier module. That is, due to the presence of the resistor, a loss occurs in the drain current of the power amplifier, resulting in an increase in the consumption current of the power amplifier module.